Quo nomine vis vocari? A random-copying model explains the temporal sequence of papal names

Original Abstract

The study of cultural evolution seeks to understand the processes by which behavioral variants are chosen in cultures over time, often as the result of large numbers of individual human choices. The selection of new popes, each of whom chooses a papal name -- typically reusing previous names in reference to previous popes -- is among the longest ongoing cultural processes taking place in a single human institution. Here, we use the record of papal names as a setting for long-term analysis of human cultural behavior. Although papal name choices are careful individual decisions, we find that the long-term sequence of papal names accords with predictions of a family of models developed in population genetics and stochastic processes -- Ewens sampling theory and the Chinese restaurant process -- which in the case of papal names amounts to randomly copying an existing name in proportion to its frequency, with the possibility of innovation of new names (mutations). Hence, despite the consideration that enters into choices of individual papal names, aggregate cultural behavior in a 2000-year old human process can potentially be described with simple laws. We discuss instances in which particular historical events might have caused temporary deviations from the random-copying model.